Walking is generally a unique orchestration of three physical properties: force, resistance and balance. Sustained walking may require that these functions occur over and over in the course of a day. The leg, ankle and foot are generally the terminus of all forces generated in the act of walking. If the forces applied to the leg, ankle and foot become too great, injury occurs. Some of the techniques used by researchers, clinicians and/or surgeons for assessing muscle strength of the leg, ankle, and/or foot are discussed hereinbelow.
Some traditional techniques available to clinicians and surgeons (e.g., podiatrist and orthopedic surgeons) for assessing conditions and defining treatment plans for their patients do not take into account the mechanical interaction between the leg, ankle, and foot, which may increase the difficulty of diagnosing and/or treating patients. As an example, clinicians have traditionally approached the ankle as a simple hinge joint and have evaluated the ankle by measuring equinus. Typically, the methods used to measure equinus focus on the range of motion of the ankle as defined by the relationship between the long axis of the foot and the long axis of the leg. Equinus may be present if dorsiflexion (i.e., bending backward) of the foot at the ankle is limited to less than ten degrees when the subtalar joint is in the neutral position and the midtarsal joint is maximally pronated. But, some clinicians and researchers have found that measuring equinus can be subjective and difficult to duplicate based upon variations in training and testing techniques. For instance, the lack of uniformity may make it difficult to determine whether surgical lengthening of the Achilles Tendon is appropriate. Moreover, measuring equinus may measure the range of motion of the ankle, but it does not measure the mechanical function of the leg, ankle and foot.
As a result, to understand the physical properties of the forces that occur during the act of walking, the leg ankle and foot should be discussed as a lever arm as these physical properties are predominately controlled by a lever arm made up of the leg, ankle and foot. However, this lever arm carries sustained loads that have been typically difficult to quantify and illusive to researchers who employ direct physical measurements. Subsequently, when clinicians and surgeons try to assess the function of this lever arm, they may be unable to use a reliable measurement of its function, which may hinder diagnosis and/or treatment of patients. Numerous attempts have been made to assess the lever arm of the leg, ankle and foot in a research setting using direct measurements, but a method applicable in a clinical setting has generally not been established. Therefore, clinicians may be unable to quantitatively assess the mechanical function of this lever arm.
In particular, critical to a discussion of lever arms are the specific characteristics of the lever arm such as the length, the location, and the center of load. The ability to quantify these physical properties of the lever arm would enable a direct measurement of the lever arm. Numerous studies have been performed to define the specific lever arm characteristics of the ankle. One study noted that surgical placement of transducers would be required to monitor direct pressure. As such, although some direct measurement studies have been performed to define the specific lever arm characteristics of the ankle, the majority of these studies are hard to interpret and are generally difficult to duplicate in a clinical setting. Moreover, a study described measuring force delivered to bone joints and ligaments as direct and indirect. Thus, indirect models may be required to measure living structures.
Another conventional technique measures the force of one muscle against another muscle, referred to as muscle strength ratios, by measuring the relationship between two reciprocal muscles, an agonist and an antagonist. Muscle strength ratios have traditionally been measured using apposing muscle groups. As an example, muscle strength ratios are commonly used to measure the hamstrings and quadriceps and this measurement creates a ratio of the muscle function of the knee. The relationship between the two muscles is generally that of eccentric vs concentric muscle action. Muscle strength ratios may also be described by a number of different names including agonist/antagonist ratios, concentric/eccentric ratios, reciprocal contraction mode ratios and/or reciprocal muscle group ratios.
Muscle strength ratios may be measured using static or dynamic muscle testing. Static muscle testing, or isometric contraction of muscle may be created when muscle is contracted with no change in the muscle's length. Isometric dynamometry measures the maximal center of pressure (COP). Maximal isometric COP measurements (or isometric dynamometry) may be performed by a number of different commercially available products. However, generally, only a limited focus of data is measured by isometric dynamometry. As such, isometric dynamometry may simply give a snapshot of the function of a lever arm, recording the maximal force generated by the lever arm.
On the other hand, dynamic muscle testing, or isokinetic dynamometry, measures the moment (torque) of a lever arm as resistance is applied at a constant speed. Isokinetic dynamometry may measure peak torque, angle specific torque, work (work=torque·times·distance), power, the rate of torque production and torque acceleration energy. Peak torque, with a relationship to the angle of the joint, is a commonly used isokinetic measure. Isokinetic assessment typically requires expensive equipment that is not readily available to most clinicians. Moreover, although an isokinetic assessment may be a more comprehensive record of the entire range of motion of a lever arm, the validity, reproducibility and clinical significance of the measures are often times questionable due to lack of standardization. As such, isokinetic dynamometry is generally used for testing muscle strength ratios in the research community but not widely used by clinicians in a clinical practice.
Several researchers have also attempted to define the percentage of load delivery by individual muscle and tendon units of the lever arm. The general rule of thumb is that a muscle's potential for work is directly proportional to the cross section of the muscle. While the contribution of each muscle and tendon unit may be defined in a lab setting, clinical application of this testing has also generally been difficult to accomplish.
A need therefore exists for a muscle strength measurement of the lever arm that may be used in a clinical setting.